zhihuang 81c3a03004 Added a callback function OnAddTrack to PeerConnectionObserver
Added the callback in native c++ API.
The callback function is called when a track is added and a new RtpReceiver is created.
The application can tell when tracks are added to the streams by listening to this callback.

BUG=webrtc:6112

Review-Url: https://codereview.webrtc.org/2505173002
Cr-Commit-Position: refs/heads/master@{#15142}
2016-11-17 20:06:37 +00:00
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